it would be interesting to be able to plot a 2D graph within an Ada code rather than having to save values say to file and use an external drawing software afterwards. A search
I'd give a try to GTKAda. I'm not completely sure this subsystem would allow you to do 2D graphs as I've never used it for that purpose, but it can be done with GTK+ (C/C++).
There is also a guide on GTKAda and a reference manual, some documentation on the GTK toolkit itself may also come handy.